1. Field of the Invention
The present invention relates to a method and apparatus for sensing remote objects. More particular, the present invention relates to sensing remotes objects using unique and random signal waveforms in both space and time.
2. Background of the Invention
Current techniques for search and track surveillance use an array of transmitter source elements (e.g., electromagnetic or acoustic) and an array of receiver sensor elements. The array of transmitter source elements generates one or more transmitted signal beams that the array of receiver sensor elements receives by forming beams, the received signals being scattered by remote objects. The received scattered signals are subsequently processed to yield direction and range information of remote objects. In many systems, the array of transmitter source elements and the array of receiver sensor elements share elements within a single array.
In the current techniques, only a limited number of signal beams can be formed simultaneously. Therefore, a remote object space (volume of coverage) must be sequentially scanned sector by sector until the complete remote object space of interest has been thoroughly scanned. A fast moving object traveling through the remote object space can escape detection simply because the signal beam is never directed toward the object at any instant. Also, in the current technique, simple countermeasures may affect the system. This is particularly true if the remote object space is sequentially scanned in the same manner repeatedly using the same signal waveform throughout the object space.
Therefore, there is a need to provide a method and apparatus for forming more beams simultaneously to reduce the likelihood that a remote object will escape detection. There is also a need to provide a method and apparatus for reducing the effects of countermeasures against the apparatus.
In accordance with the invention, there is provided an apparatus having a transmitter and a receiver for sensing remote objects. The transmitter comprises J transmitter source elements. The apparatus further comprises a source generator for providing J equal carrier signals and a modulator signal generator for generating J statistically independent chip sequences. Each chip sequence comprises a plurality of chips, each chip having a random phase. Still further, the apparatus comprises a modulator for independently modulating the J equal carrier signals with the J statistically independent chips sequences, respectively, to generate J modulated signals. Each modulated signal comprises a plurality of chips, each chip having a random phase. The J modulated signals are transmitted by the J transmitter source elements, respectively, forming a composite signal beam. The transmitter source elements are separated by approximately xc2xd wavelength.
There is also provided a method for sensing remote objects comprising the steps of generating J equal carrier signals and J statistically independent chip sequences. Each chip sequence comprises a plurality of chips, each chip having a random phase. The method further comprises independently modulating the J equal carrier signals with the J statically independent chip sequences, respectively, to generate J modulated signals and then transmitting the J modulated signals, forming a composite signal beam. Each modulated signal comprises a plurality of chips, each chip having a random phase.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.